All TwiddleFactor multichannel apps automatically detect the current input and output route(s). The built-in microphone and speaker of your iPad/iPhone are the default input and output. The built-in microphone only enables mono audio input. But the Audio IO settings provide the possibility to select the built-in microphone and polar pattern (options depend on your device). However, it is recommended to use the default if you do not aim for something specific.
External USB-audio-interfaces (e.g. Alesis, RME Fireface) or stereo microphones (TASCAM, Røde) provide up to 8 input channels, all of which are displayed with separate instruments on the main screen.
Each channel can be enabled or disabled separately. Just tap the circle representing the channel. Channels of each sources are always named L (for left), R (for right), 3, 4, 5, … and so.
You can use Audiobus to add additional input sources. Currently Audiobus supports up to three inputs simultaneously.
Please have a look at the support article about Audiobus for further information.